For dense array style circuit layouts, such as static random access memories (SRAMs), a small cell is desired. In multi gate field effect transistor (MuGFET) technologies, a core cell area is constrained by the distance between two fins (fin pitch) and the number of fins used per device. There is a desire to reduce the area needed for core cells using MuGFET technologies.
Two fabrication processes have been used to produce fins that yield a small distance between fins, referred to as fin pitch. In one method, multiple sacrificial spacers are first formed. Fins are then formed on the sides of the spacers. This results in an even number of fins. A similar process using altPSM (alternating phase shift modulation) where each fin has to contain a boundary between a 0° and 180° phase shifting area also results in an even number of fins.
These processes provide a small fin pitch. Each fin is defined by a spacer or closed boundary in the case of PSM. For convenience, the term spacer is used, but also applies to the PSM method. It is therefore not possible to build fins in-between two spacer structures. Thus, it is not possible to build transistor stacks consisting of transistors with an odd number of fins. Single fin devices require the use of additional complex clean-out steps, resulting in inefficient use of chip area.